Click Design > Backgrounds, and click the style you want. Even if it’s the same background style as before, assigning it again should match up the sizes.

Why does this size mismatch happen? It’s usually because one background page is shared by multiple pages in a drawing. The background takes the size of the largest diagram in those pages, and uses that size with each foreground page.

Multiple pages can use the same background image, but it’s best to give each drawing page its own background page, using the procedure above.